The present invention relates to a method for insitu remediation of soil.
Removal of contamination from surface and near-surface is often accomplished, by physically removing contaminated soils and subsequently treating the soils at elevated temperatures to vaporize and/or destruct the contamination. This is very expensive, and also exposes the contaminants to the atmosphere as the contaminated soil is being removed and handled. Insitu methods have therefore been proposed, both to reduce the costs of removing the contaminants and as a method to reduce the exposure to the contaminants. Heating soil by conduction from heater wells has been proposed, and a well heater useful in such a process is disclosed in U.S. Pat. No. 5,190,405. This well heater functions as both a means to inject heat, and as a means to remove vapors from the soil. A flexible sheet is placed over the surface in order to seal the surface to vapors that may be drawn into the soils from the surface (and impede the heating process) and to keep water from entering the soil from the surface. Insulation is placed on top of the flexible sheet to reduce heat loss from the system. The flexible sheet of this system must be capable of withstanding the temperatures required for removal of contamination at the surface. If contamination originated from below the surface, such as a buried tank or pipe, then surface temperatures below those required for removal of contaminants may be acceptable, but in many situations, surface temperatures are required that are well above temperatures that can be tolerated by silicone sheets or other pliable sheets. It is desirable to have a heater well design which is tolerant of such high surface temperatures.
U.S. Pat. No. 5,244,310 discloses a system for remediation of near-surface contamination using shallow heater and vapor extraction wells wherein the wells are placed through a frame. An insulated shield covers the frame. This system utilizes the proximity of the vapor extraction wells to the heat injection wells to ensure removal of the contaminants, and does not include any particular seal at the surface to ensure that excessive amounts of air do not penetrate the surface and keep the soil from reaching acceptable remediation temperatures. Air leaking into the contaminated soil also reduces the vacuum that can be applied. A lower pressure within the soil results in more efficient removal of contaminants. It would be preferable to have such a system wherein the surface is sealed in order to both eliminate air intrusion, which cools the soil and increases the amount of vapor to be treated by the vapor extraction system, and also to result in a lower pressure within the soil.
It is therefore an object of the present invention to provide a method for remediation of near-surface contaminants in which the surface is sealed to prevent significant intrusion of air into the contaminated soil. It is another object of the invention to provide such a method wherein heater-vapor extraction wells can be provided at a relatively low cost.
These and other objects are provided by a method to remove contaminants from near-surface soil, the method comprising the steps of: placing a plurality of metal sheets on the surface above the contaminated soil to form a contiguous cover over at least a portion of the contaminated soil; seal welding adjacent metal sheets together; cutting a plurality of holes in the metal sheet; providing a plurality of heater wells and a plurality of vapor extraction wells into the contaminated soil through the holes cut in the metal sheets; providing vapor extraction wellheads over the vapor extraction wells, the vapor extraction wellheads sealed to the metal sheets; and removing contaminants from the near-surface soil by heating from the heater wells and extracting vapor through the vapor extraction wells. The wells are preferably both heater wells and vapor extraction wells, and the wellheads are preferably bolted to a flange which is welded to the metal sheets to ensure a positive seal at the surface. Insulation is preferably placed over the metal sheets prior to heating of the soil, and a rain cover placed over the insulation to keep rain out of the insulation and away from the soil being heated. The edges of the contiguous cover is preferably sealed by a vertical metal sheet placed into the soil, with a 90 degree angled lip on top, and the lip attached to the edge of the continuous sheet to form the seal.
The method of the present invention forms a relatively inexpensive apparatus for decontamination of soils in a fast and energy efficient manner.